A micro perifusion device for pancreatic islets allowing concomitant recordings of intermediate metabolites and insulin release

The anti-CD40L monoclonal antibody AT-1501 promotes islet and kidney allograft survival and function in nonhuman primates

Prior studies of anti-CD40 ligand (CD40L)-based immunosuppression demonstrated effective prevention of islet and kidney allograft rejection in nonhuman primate models; however, clinical development was halted because of thromboembolic complications. An anti-CD40L-specific monoclonal antibody, AT-1501 (Tegoprubart), was engineered to minimize risk of thromboembolic complications by reducing binding to Fcγ receptors expressed on platelets while preserving binding to CD40L. AT-1501 was tested in both a cynomolgus macaque model of intrahepatic islet allotransplantation and a rhesus macaque model of kidney allotransplantation. AT-1501 monotherapy led to long-term graft survival in both islet and kidney transplant models, confirming its immunosuppressive potential. Furthermore, AT-1501-based regimens after islet transplant resulted in higher C-peptide, greater appetite leading to weight gain, and reduced occurrence of cytomegalovirus reactivation compared with conventional immunosuppression. These data support AT-1501 as a safe and effective agent to promote both islet and kidney allograft survival and function in nonhuman primate models, warranting further testing in clinical trials.

The Effect of Recovery Warm-up Time Following Cold Storage on the Dynamic Glucose-stimulated Insulin Secretion of Isolated Human Islets

Standardized islet characterization assays that can provide results in a timely manner are essential for successful islet cell transplantation. A critical component of islet cell quality is β-cell function, and perifusion-based assessments of dynamic glucose-stimulated insulin secretion (GSIS) are the most informative method to assess this, as they provide the most complex in vitro evaluation of GSIS. However, protocols used vary considerably among centers and investigators as they often use different low- and high-glucose concentrations, exposure-times, flow-rates, oxygen concentrations, islet numbers, analytical methods, measurement units, and instruments, which result in different readouts and make comparisons across platforms difficult. Additionally, the conditions of islet storage and shipment prior to assessment may also affect islet function. Establishing improved standardized protocols for perifusion GSIS assays should be an integral part of the ongoing effort to increase the rigor of human islet studies. Here, we performed detailed evaluation of GSIS of human islets using a fully automated multichannel perifusion instrument following various warm-up recovery times after cold storage that corresponds to current shipping conditions (8°C). We found that recovery times shorter than 18 h (overnight) resulted in impaired insulin secretion. While the effects were relatively moderate on second-phase insulin secretion, first-phase peaks were restored only following 18-h incubation. Hence, the biphasic profile of dynamic GSIS was considerably affected when islets were not allowed to recover for a sufficient time after being maintained in cold. Accordingly, while cold storage might improve islet cell survival during shipment and prolong the length of culture, functional assessments should be performed only after allowing for at least overnight recovery at physiological temperatures.

Concentration-Dependency and Time Profile of Insulin Secretion: Dynamic Perifusion Studies With Human and Murine Islets

The detailed characterization and quantification of the kinetics of glucose-stimulated insulin secretion (GSIS) by normal pancreatic islets is of considerable interest for characterizing β-cell dysfunction, assessing the quality of isolated islets, and improving the design of artificial pancreas devices. Here, we performed dynamic evaluation of GSIS by human and mouse islets at high temporal resolution (every minute) in response to different glucose steps using an automated multichannel perifusion instrument. In both species, insulin responses were biphasic (a transient first-phase peak followed by a sustained second-phase), and the amount of insulin released showed a sigmoid-type dependence on glucose concentration. However, compared to murine islets, human islets have (1) a less pronounced first-phase response, (2) a flat secretion rate during second-phase response, (3) a left-shifted concentration response (reaching half-maximal response at 7.9 ± 0.4 vs. 13.7 ± 0.6 mM), and (4) an ~3-fold lower maximal secretion rate (8.3 ± 2.3 vs. 23.9 ± 5.1 pg/min/islet at 30 mM glucose). These results can be used to establish a more informative protocol for the calculation of the stimulation index, which is widely used for islet assessment in both research and clinical applications, but without an accepted standard or clear evidence as to what low- to high-glucose steps can provide better characterization of islet function. Data obtained here suggest that human islet functionality might be best characterized with a dynamic stimulation index obtained with a glucose step from a low of 4-5 to a high of 14-17 mM (e.g., G4 → G16).

Automated, High-Throughput Assays for Evaluation of Human Pancreatic Islet Function

An important challenge in pancreatic islet transplantation in association with type 1 diabetes is to define automatic high-throughput assays for evaluation of human islet function. The physiological techniques presently used are amenable to small-scale experimental samples and produce descriptive results. The postgenomic era provides an opportunity to analyze biological processes on a larger scale, but the transition to high-throughput technologies is still a challenge. As a first step to implement high-throughput assays for the study of human islet function, we have developed two methodologies: multiple automated perifusion to determine islet hormone secretion and high-throughput kinetic imaging to examine islet cellular responses. Both technologies use fully automated devices that allow performing simultaneous experiments on multiple islet preparations. Our results illustrate that these technologies can be applied to study the functional status and explore the pharmacological profiles of islet cells. These methodologies will enable functional characterization of human islet preparations before transplantation and thereby provide the basis for the establishment of predictive tests for β-cell potency.

Isolation of Pancreatic Islets from Nonhuman Primates

Nonhuman primates (NHP) constitute a highly relevant pre-clinical animal model to develop strategies for beta cell replacement. The close phylogenetic and immunologic relationship between NHP and humans results in cross-reactivity of various biological agents with NHP cells, as well as a very similar cytoarchitecture between islets from human and NHP that is strikingly different from that observed in rodent islets. The composition and location of endocrine cells in human or NHP islets, randomly distributed and associated with blood vessels, have functional consequences and a predisposition for paracrine interactions. Furthermore, translation of approaches that proved successful in rodent models to the clinic has been limited. Consequently, data collected from NHP studies can form the basis for an IND submission to the FDA. This chapter describes in detail the key aspects for isolation of islets from NHP, from organ procurement up to assessment of islet function, comparing and emphasizing the similarities between isolation procedures for human and NHP islets.

Immune cell populations in nonhuman primate islets

Islet transplantation is a promising cellular therapy for the treatment of type 1 diabetes (T1D). The immunogenicity of isolated islets has been of interest to the transplant community for many years, as upon transplantation, islets are damaged or destroyed through specific and nonspecific inflammatory and immune events. Antigen presenting cells (APC) are crucial intermediates in the generation of both innate and specific immune responses, and it has long been understood that some APC are resident in islets in situ, as well as after isolation. Our aim was to identify and characterize intraislet resident populations of APC and other immune cells in islets from nonhuman primates (Macaca fascicularis) in situ (pancreas biopsies obtained prerecovery) and after isolation using immunohistochemistry, confocal microscopy, and flow cytometry. The numbers of cells obtained in situ are similar to those in islets postisolation. Each isolated islet equivalent contains an average of 21.8 immune cells, 14.7 (67%) of which are APC. Many of these APC are dentritic cells and, surprisingly, 50% are B lymphocytes. The number of islet-resident immune cells increases with islet size, with greater numbers in large versus small islets (p < 0.001). The APC were localized around the exterior or spread evenly throughout the islets, with no definitive orientation identified. This knowledge will be useful to develop tailored modulation strategies to decrease immunogenicity, enhance engraftment, and ultimately prevent islet rejection.

Long-term survival of nonhuman primate islets implanted in an omental pouch on a biodegradable scaffold

The aim of this study was to test whether an omental pouch can be used as an alternative site for islet implantation in diabetic monkeys. Here we report the successful engraftment of islets in diabetic cynomolgus monkeys when loaded on a synthetic biodegradable scaffold and placed in an omental pouch. One autologous and five allogeneic diabetic monkey transplants under the cover of steroid-free immune suppression (SFIS) were undertaken. Fasting blood glucose (FBG) and C-peptide (CP), exogenous insulin requirements (EIR), intravenous glucose tolerance test (IVGTT), A1C and histopathology were used to assess islet engraftment and survival. All animals achieved CP levels > 1.0 ng/mL following transplant, a 66-92% posttransplant decrease in EIR and reduced A1C. Following graft removal, CP became negative and histopathological analysis of the explanted grafts demonstrated well-granulated and well-vascularized, insulin-positive islets, surrounded by T-cell subsets and macrophages. Compared to intrahepatic allogeneic islet transplants (n = 20), there was a delayed engraftment for omental pouch recipients but similar levels of CP production were ultimately achieved, with a broad range of IEQ/kg transplanted in both sites. Our results suggest this extrahepatic transplantation site has potential as an alternative site for clinical islet cell transplantation.

The control of insulin release by sugars

The interaction of glucose, the major physiological regulator of insulin secretion, with the beta-cell involves the recognition of glucose as a signal, the transduction of this recognition into an intracellular event and the coupling of the event to the exocytotic discharge of insulin from secretory granules. The following aspects of this system are discussed: (1) the mechanism of insulin release; (2) the evidence implicating Ca2+ and cyclic AMP as coupling factors; (3) the main characteristics of glucose-stimulated insulin release; (4) gluco-receptor models and the evidence for them; (5) possible mechanisms for transduction of the response to glucose; (6) the extent to which the systems of the secretory response to sugars may also be involved in the control of proinsulin biosynthesis; (7) whether starvation induces specific changes in the glucoreceptor system.

Interference with tissue factor prolongs intrahepatic islet allograft survival in a nonhuman primate marginal mass model

BACKGROUND

Tissue factor (TF) expression on islets can result in an instant blood-mediated inflammatory reaction (IBMIR) that contributes to early islet loss. We tested whether peritransplant protection of islets from IBMIR with a monoclonal anti-TF antibody (CNTO859) would enhance engraftment in our nonhuman primate marginal mass model.

METHODS

Each of six pairs of cynomolgus monkeys (CM) with streptozotocin-induced diabetes was closely matched for metabolic control and was transplanted with 5,000 IEQ/kg allogeneic, ABO-compatible islets from the same donor under the cover of steroid-free immunosuppression. For each pair, experimental animals received islets cultured with 20 microg/mL anti-TF and were dosed with 6 mg/kg anti-TF intravenously, 10-25 min before islet infusion; control monkeys received an equal number of islets from the same preparation cultured without anti-TF and no in vivo treatment.

RESULTS

Early fasting C-peptide (CP) values were different between (P<0.01), but not within, pairs and correlated with in vitro functional capacity of islets as assessed by perifusion (r=0.60; P=0.022). Compared to their matched controls, experimental animals had decreased posttransplant markers of coagulation, higher fasting CP levels (1 month posttransplant and end of study) and prolonged graft function.

CONCLUSIONS

These data suggest that pretreatment of islets and the recipient with anti-TF may limit the effects of IBMIR, thereby enhancing islet engraftment and survival.

A buffer temperature controlled perifusion system to study temperature dependence and kinetics of insulin secretion in MIN6 pseudoislets

INTRODUCTION

The perifusion of pancreatic islets is a well-known method to investigate the kinetics of insulin secretion. Nevertheless, little interest has been attributed to a precise temperature control in perifusion systems. Insulin secretion from MIN6 cells, cultured as monolayers, differs substantially from pancreatic islets, at least partly due to missing beta-to-beta cell contacts. These cellular contacts are abundant in MIN6 pseudoislets, which show a more pronounced glucose-induced insulin release. Here, a perifusion system that directly and dynamically controls the perifusion buffer temperature inside the reaction chamber is described. Additionally, the influence of small temperature changes, glucagon-like peptide 1 (GLP-1) and tolbutamide on insulin release from MIN6 pseudoislets is examined.

METHODS

MIN6 cells were cultured in suspension culture dishes to generate MIN6 pseudoislets. The pseudoislets were perifused using a newly developed 12-channel perifusion system. The buffer temperature inside the reaction chambers was dynamically controlled by a programmable proportional plus integral plus differential (PID) controller. Insulin was determined by radioimmunoassay.

RESULTS

After adjusting the PID controller, the temperature inside the reaction chambers was constant in a very narrow range. The first phase of the glucose-induced insulin secretion was enhanced from 1.0+/-0.1 to 2.8+/-0.2 ng insulin/ml and the second phase from 5.4+/-0.9 to 17.8+/-1.3 ng insulin/ml, when the temperature was elevated by 1 degrees C, from 37 to 38 degrees C. GLP-1 concentration dependently increased insulin release at 15.0 mM and was ineffective at 0.0 mM glucose. Tolbutamide induced a concentration-dependent increase in both phases of the insulin secretion.

DISCUSSION

MIN6 pseudoislets are a useful tool to study insulin secretion from beta-cells, which are arranged in clusters like pancreatic beta-cells in the islet. The strong influence of temperature on insulin release from these pseudoislets requires a perifusion system, which precisely controls the buffer temperature.

Insulin secretion in pancreatic islets from rats with cirrhosis

Cirrhosis was induced in rats by subcutaneous injections of CCl4 for 13 or 17 weeks. The morphology of the pancreatic islets from the CCl4-treated rats was found to be normal. The CCl4-treated rats had lower fasting serum glucose levels and higher serum insulin levels than the controls. After an oral glucose load (3 g/kg body weight), glucose levels in CCl4-treated rats stayed within the normal range, whereas the serum insulin levels remained higher with a delayed decline of insulin with time. In vitro perifusion of islets from the CCl4-treated rats showed that the response to 16.7 mmol/l glucose was reduced with both lower total insulin output and stimulated insulin output, whereas the patterns of first and second phase of insulin release did not differ. The insulin content of the perifused islets was not affected by 13 weeks of CCl4 treatment. Islets from rats treated with CCl4 for 17 weeks showed normal secretory response to 20 mmol/l L-arginine. Taken together, the results, showing normal or reduced capacity for insulin secretion, suggest that the hyperinsulinemia accompanying CCl4-induced cirrhosis is not due to increased secretion of the pancreatic islets. It may rather be associated with decreased insulin degradation by the liver with cirrhosis.

Effect of arginine and glucagon on perifused purified beta cells

The secretory response of purified beta cells was studied employing a newly developed microperifusion technique. In control experiments, initial high insulin release up to 50 minutes followed by a steady state basal level was observed up to five hours. The changes in hydrodynamic pressure of perifusion produced small, statistically non-significant fluctuations in the insulin release in succeeding two samples of effluent. Pulse stimulation of the purified beta cells with arginine 10 mM and 20 mM produced a biphasic secretory response. The total insulin secretion with arginine (1, 5, 10, or 20 mM) was concentration dependent. Glucagon (1, 5, 10, or 20 nM) produced concentration dependent stimulation of insulin secretion. The results of these studies indicate that the secretory responses of the purified beta cells could be studied employing the microperifusion technique described in this report. The purified beta cells respond to arginine and glucagon with increased insulin secretion as observed in microperifusion and static incubation studies (1, 2).

Pepsinogen secretion from perifused frog peptic glands: rapid transients detected with a modified pepsin assay

In order to define the early transient responses of peptic cells to stimulation, we have modified the acidified hemoglobin substrate digestion method for pepsin to provide a very sensitive, linear (10-250 ng/ml), reproducible, inexpensive (less than 3 cents/sample) and simple semi-automated assay. Using a specially designed perifusion chamber, this method was used to accurately measure secretion at 1 min intervals from approximately 3 mg of isolated peptic glands from the esophageal peptic organ of R. catesbeiana, containing approximately 150 micrograms pepsinogen. Responses to carbachol applied for 1, 2 and 4 min could be described in discrete 1 min intervals. Secretion stimulated by carbachol (1-2 min) peaked at 200-300% of basal and upon withdrawal decayed with t1/2 approximately 3 min. Atropine added to continuous carbachol stimulation inhibited secretion with t1/2 approximately 4 min, indicating rapid metabolism of activated messengers.

L-leucine methyl ester stimulates insulin secretion and islet glutamate dehydrogenase

Column perifusion of collagenase-isolated mouse pancreatic islets was used to study the dynamics of insulin release in experiments lasting for several hours. The methyl esters of L-leucine and L-arginine were synthesized. Whereas L-arginine methyl ester (L-arginine OMe) had no effect, L-leucine OMe stimulated the release of insulin. The effect of L-leucine OMe was maximal at 5 mmol/liter. Whereas the Km for glucose-stimulated insulin release was unaffected by 1 mmol/liter L-leucine OMe, the maximal release of D-glucose was increased by the amino acid derivative that appeared more effective than L-leucine. L-Leucine OMe was also a potent stimulus of insulin release from the perfused mouse pancreas. In the presence of 10 mmol/liter L-glutamine, 1 mmol/liter L-leucine OMe induced a 50- to 75-fold increase in insulin release. A similar stimulatory effect was also observed in column-perifused RIN 5F cells, a cloned rat islet tumor cell line. A twofold increase in islet glutamate dehydrogenase activity was induced by 5 mmol/liter L-leucine OMe, a larger effect than that of L-leucine (P less than 0.02), whereas L-arginine OMe had a small inhibitory effect. We conclude that L-leucine OMe is a potent stimulus of insulin secretion and that its effect on the beta-cells may be exerted by activating islet glutamate dehydrogenase.

Effects of alpha-adrenoceptor agonists and antagonists on insulin secretion, calcium uptake, and rubidium efflux in mouse pancreatic islets

Epinephrine, norepinephrine or the more selective alpha-2 adrenoceptor agonist, clonidine, inhibited insulin release from isolated pancreatic islets of lean mice or obese mice homozygous for the gene ob. Clonidine was highly effective at 0.1 mumol/l. In contrast, the preferential alpha-1 adrenoceptor agonist, phenylephrine, had no or only a modest effect at 10 mumol/l. The effects of norepinephrine or clonidine were counteracted by yohimbine, a preferential blocker of alpha-2 receptors, but not by prazosine, an alpha-1 receptor blocker. The glucose-stimulated uptake of 45Ca2+ in the islets was only consistently inhibited by epinephrine. This effect was counteracted by yohimbine. Clonidine had no effect on the release of 86Rb+ from preloaded islets. It is concluded that insulin secretion is suppressed by alpha-2 receptor agonism in the pancreatic beta-cells and that this effect is mediated by mechanisms other than the transmembrane fluxes of calcium or potassium ions.

Stimulation of the insulin secretory mechanism following barium accumulation in pancreatic beta-cells

Electrothermal atomic absorption spectroscopy was employed for measuring barium in beta-cell-rich pancreatic islets microdissected from ob/ob-mice. Both the uptake and efflux of barium displayed two distinct phases. There was a 4-fold accumulation of barium into intracellular stores when its extracellular concentration was 0.26 mM. Unlike divalent cations with more extensive intracellular accumulation, the washout of Ba2+ was not inhibited by D-glucose. Ba2+ served as a substitute for Ca2+ both in maintaining the glucose metabolism after removal of extracellular Ca2+ and making it possible for glucose to stimulate insulin release. Furthermore, Ba2+ elicited insulin release in the absence of glucose and other secretagogues. The latter effect was reversible and was markedly potentiated under conditions known to increase the beta-cell content of cyclic AMP. It is likely that the observed actions of Ba2+ are mediated by Ca2+, since Ca2+ -dependent regulatory proteins, such as calmodulin, apparently cannot bind Ba2+ specifically.

Characterization of the rat parotid beta-adrenoceptor

1 The effects of various beta-adrenoceptor agonists on amylase secretion from the rat parotid gland were studied by means of two different in vitro techniques. 2 The dose-response relation for each agonist was established, as also were the ED50 values. 3 All drugs appeared to act directly on the acinar cells, as reserpine-treatment did not abolish their secretagogic effects. 4 Two groups of agonists could be distinguished: one group consisting of adrenaline, noradrenaline and the B1-selective agonist prenalterol (H133/22) with a high enzyme discharge potency and a second group consisting of the beta 2-agonists, terbutaline and salbutamol, with a markedly lower effect. 5 The present data further support the theory that rat parotid acinar cells are supplied mainly with beta-adrenoceptors of the beta 1-subtype, similar to those present in heart and adipose tissue.

Calcium and pancreatic beta-cell function. The mechanism of insulin secretion studied with the aid of lanthanum

La3+ was used to study the involvement of Ca2+ in insulin secretion in beta-cell-rich pancreatic islets micro-dissected from non-inbred ob/ob mice. Ultrastructural studies revealed that the localization of La3+ was entirely restricted to the exterior of the cells. Consistent with a membrane action, exposure to La3+ failed to affect glucose oxidation and either the sucrose space or the general ultrastructure of the islets. In contrast, La3+ had marked effects on insulin release and 45Ca fluxes. Exposure to La3+ resulted in pronounced inhibition of insulin release irrespective of the presence or absence of Ca2+, 3-isobutyl-1-methylxanthine or glucose. Perifusion experiments revealed that the inhibitory action was prompt, sustained and readily reversible. Removal of La3+ was associated with a subsequent prolonged stimulatory phase of insulin release even in medium deficient in Ca2+. This action could not be attributed to an increase in cyclic AMP, but was potentiated by 3-isobutyl-1-methylxanthine and abolished by L-adrenaline. La3+ displaced 45Ca from superficially located binding sites and inhibited the uptake and efflux of 45Ca. The stimulatory and inhibitory actions of glucose on 45Ca efflux were also abolished in the presence of 2 mM-La3+ Removal of La3+ was associated with the preferential mobilization of 45Ca incorporated in response to glucose. The results indicate that binding of La3+ to superficial sites in the plasma membrane leads to inhibition of insulin release by suppression of transmembrane Ca2+ fluxes. It is suggested that accumulation of Ca2+ in the cytoplasm accounts for the stimulation of insulin release seen after removal of La3+ from inhibitory binding sites in the beta-cell plasma membrane.

Ca2+ transport in pancreatic beta-cells during glucose stimulation of insulin secretion

The role of Ca2+ in the regulation of insulin secretion was evaluated using beta-cell-rich pancreatic islets isolated from ob/ob-mice. The glucose stimulation of the secretory activity is supposed to result from accumulation of Ca2+ in the submembrane cytoplasmic space. It is likely that this process reflects the balance between increased entry of Ca2+ into the beta-cells and an enhanced sequestration of Ca2+ in the organelle sinks. The proposed model can explain the cAMP potentiation of glucose-stimulated insulin release with suppression of the mitochondrial Ca2+ uptake. Furthermore, differences in the Ca2+ buffering capacity of the secretory granules may account for other characteristic features of glucose-stimulated insulin release, in particular its biphasic nature and sensitivity to suppression on withdrawal of nutrients.

Dynamics of beta adrenoceptor induced amylase release and cyclic AMP accumulation in the guinea pig submandibular gland

The dynamics of amylase release from the guinea pig submandibular gland were studied in vitro by applying a multi-channel microperfusion set. This technique makes it possible to measure time related enzyme release more accurately and to take samples of perifused tissue at short intervals. Stimulation of the beta-adrenoceptor with norepinephrine gives rise to a rapid initial enzyme discharge, detectable within 15 s. Administration of propranolol inhibits enzyme release, which is not restored after removal of the agent. Simultaneous measurements of tissue cyclic AMP during norepinephrine stimulation at various time intervals display a significant increase of cAMP as early as 15 s after stimulation of secretion. This increase of cAMP thus coincides with the discharge of amylase. In addition, cAMP continuously accumulates during 30 min of norepinephrine perifusion of the slices. The present study describes a valuable tool with high sensitivity for visualizing the relations between enzyme secretion from the salivary gland and the intracellular biochemical processes. The data obtained further indicate a close correlation between amylase and cAMP during the initial phase of enzyme discharge.

The effect of calcium on somatostatin inhibition of insulin release and cyclic AMP production in mouse pancreatic islets

The effect of somatostatin on glucose-induced insulin secretion and cyclic AMP accumulation in isolated islets from obese, hyperglycemic ob/ob mice was studied in a microperifusion system. The normal biphasic pattern of insulin release as well as the inhibitory pattern of insulin release produced by somatostatin (0.5--1 microgram/ml) was matched by similar changes in the intracellular concentration of cyclic AMP. When islets were stimulated by glucose (3 mg/ml) plus 3-isobutyl-1-methylxanthine (0.1 mM), somatostatin (0.5 microgram/ml) failed to inhibit insulin secretion or cyclic AMP formation in the second phase whereas in the first phase both parameters were significantly reduced by somatostatin (0.5 microgram/ml). In batch-type incubations it was shown that addition of excess calcium (to 6 mM) reversed this inhibition. In the second phase calcium potentiated the (glucose + 3-isobutyl-1-methylxanthine)-stimulated insulin secretion without affecting the cyclic AMP production. This potentiation was inhibited by somatostatin (0.1 microgram/ml). Somatostatin (1 microgram/ml) inhibited adenylate cyclase activity in islet homogenates. No effect of somatostatin on islet glucose utilization could be demonstrated. The results indicate a dual action of somatostatin in the inhibition of insulin release, one involving the islet adenylate cyclase and one affecting the islet uptake of calcium.

Calcium and pancreatic beta-cell function. I. Stimulatory effects of pentobarbital on insulin release

Islets microdissected from ob/ob-mice were exposed to 3mM pentobarbital in media which were normal or deficient in Ca2+. This treatment resulted in marked decrease of the islet content of cyclic AMP recorded in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Pentobarbital had a dual effect on insulin release. In addition to being a potent inhibitor of glucose-stimulated insulin release in media containing 2.56 mM Ca2+ it increased the amounts of insulin released in high glucose media deficient in Ca2+. There was a transient stimulation with ordinary concentrations of Ca2+ and 3mM glucose whtn the media also contained 3-isobutyl-1-methylxanthine. The stimulatory effect of pentobarbital persisted after replacing part of the Ca2+ in the beta-cell membrane with lanthanum ions and it could not be mimicked by lowering the oxygen tension of the incubation medium. It is suggested that pentobarbital stimulation of insulin release is the result of a specific action of the drug on the distribution of Ca2+ within the pancreatic beta-cells.

Effects of various modifiers of insulin release on the lanthanum-nondisplaceable 45Ca2+ uptake by isolated pancreatic islets

The uptake of 45Ca2+ by a lanthanum-non-displaceable pool in pancreatic islets was studied; Raising the extracellular D-glucose concentration from 3 to 20 mM stimulated the 45Ca2+ uptake in hand-dissected islets of ob/bo-mice as well as in collagenase-isolated islets of ob/ob or normal mice. The effect was dose-dependent in the range of 0-20 mM D-glucose and was seen throughout a wide range of extracellular calcium concentrations (16 mumol-2.56 mmol of Ca2+ added per litre of medium). The 45Ca2+ uptake was also enhanced by other known insulin secretagogues (D-mannose, L-leucine, tolbutamide) and was uninfluenced by compounds lacking insulin-releasing capacity (3-O-methyl-D-glucose, L-glucose, D-galactose, D-leucine). The stimulatory effect of D-glucose was blocked by inhibitors of glucose-induced insulin release (D-mannoheptulose, diazoxide, L-adrenaline). The results support the view that the lanthanum-nondisplaceable calcium pool is related to the insulin-releasing mechanism, although the exact nature of this relationship is still unclear.

Effects of dextran-linked chloromercuribenzoic acid on insulin release from microdissected pancreatic islets

Insulin release in response to dextran-linked p-chloromercuribenzoic acid was studied in microdissected pancreatic islets of non-inbred ob/ob-mice. No contamination of the dextran-linked mercurial with free chloromercuribenzoic acid was detected before or after the incubation with islets. In comparison with free mercurial, of the same thiol-blocking activity, the dextran-linked compound had a weak insulin-releasing action with a different dose vs. response relationship. The dextran-linked mercurial had no demonstrable effect on the islet content of cyclic AMP. The results support the hypothesis that free organic mercurials mainly stimulate insulin release by blocking thiol ground that are embedded within the beta-cell plasma membranes beneath their surfaces.

The dynamics of insulin release from mouse pancreatic islet cells in suspension

The overall dynamics of glucose-induced insulin release was strikingly similar in dispersed cells and intact islets perifused in parallel. Both preparations exhibited a latency of 1-2 min, after which period there was a brisk rise of insulin release followed by a sustained second phase. During the second phase, insulin release from dispersed cells attained a stable plateau rate, whereas the release from intact islets continued to rise. Epinephrine (1 muM) inhibited the release in both preparations, but the return to basal rate was faster in the dispersed cells than in the intact islets. The dispersed cells oxidized glucose at a constant rate for at least 60 min; the glucose oxidation was markedly sensitive to changes of the glucose concentration in the range of 3-20 mM.

Insulin secretion: mechanism of the stimulation by glucose

Glucose is one of the substrates that is controlled with the most efficient hormonal mechanisms in higher organisms. The presence of tissues such as the central nervous system which, under normal conditions, depend solely on glucose as substrate, and the sporadic type of food intake with periods of fasting of various lengths in the mammalians necessitate that the distribution of energy-rich substrates among various tissues be continuously adjusted by changes in the secretion of a number of hormones. The efficiency of this system is evidenced by the stability of the blood glucose level in man, in whom after a carbohydrate-rich meal more than 70% of the glucose that has been ingested will be retained in the liver during a single passage of portal blood, resulting in only small changes of the glucose concentration in peripheral blood. Likewise, periods of fasting up to24–36 h are followed by modest to minimal reductions of the blood glucose level, the liver now supplying the circulation with the hexose.

The pancreatic beta-cell recognition of insulin secretagogues: does cyclic AMP mediate the effect of glucose?

Insulin release and the content of cAMP were studied in microdissected pancreatic islets of noninbred ob/ob (obese) mice. In the absence of 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, 20 mM glucose had no effect on cAMP save a very small initial rise detectable by a freeze-stop perifusion technique only. However, combined with this methylxanthine, 20 mM glucose produced significant increases of cAMP both in perifused islets and in islets conventionally incubated in closed vials. Glucose shared this capacity to raise the cAMP level with D-glyceraldehyde and 1,3-dihydroxyacetone. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin, an activator of adenylate cyclase, also increased the islet cAMP level; the effects of the methylxanthine, whether or not combined with cholera toxin, were potentiated by glucose. Isobutylmethylxanthine (0.05-1.0 mM) or 5 mug/ml of cholera toxin potentiated insulin release in response to 20 mM glucose. However, only 0.5-1.0 mM isobutylmethylxanthine stimulated insulin release in the presence of 3 mM glucose, whereas 0.05-0.1 mM isobutylmethylxanthine or 5 mug/ml of cholera toxin had no effect on secretion at the low glucose concentration. These discrepancies between cAMP-promoting and insulin-releasing activities suggest that glucose does not initiate insulin release by activating the beta-cell adenylate cyclase. By being metabolized in the beta-cells, glucose may both create a release-initiating signal not identical with cAMP and enhance cAMP formation, leading to potentiation of the effect of the initiator signal.

The pancreatic beta-cell recognition of insulin secretagogues. Effects of calcium and sodium on glucose metabolism and insulin release

The transport and oxidation of glucose, the content of fructose 1,6-diphosphate, and the release of insulin were studied in microdissected pancreatic islets of ob/ob mice incubated in Krebs-Ringer bicarbonate medium. Under control conditions glucose oxidation and insulin release showed a similar dependence on glucose concentration with the steepest slope in the range 5-12mm. The omission of Ca(2+), or the substitution of choline ions for Na(+), or the addition of diazoxide had little if any effect on glucose transport. However, Ca(2+) or Na(+) deficiency as well as diazoxide (7-chloro-3-methyl-1,2,4-benzothiadiazine 1,1-dioxide) or ouabain partially inhibited glucose oxidation. These alterations of medium composition also increased the islet content of fructose 1,6-diphosphate, as did the addition of adrenaline. Phentolamine [2-N-(3-hydroxyphenyl)-p-toluidinomethyl-2-imidazoline] counteracted the effects of adrenaline and Ca(2+) deficiency on islet fructose 1,6-diphosphate. After equilibration in Na(+)-deficient medium, the islets exhibited an increase in basal insulin release whereas the secretory response to glucose was inhibited. The inhibitory effects of Na(+) deficiency on the secretory responses to different concentrations of glucose correlated with those on (14)CO(2) production. When islets were incubated with 17mm-glucose, the sudden replacement of Na(+) by choline ions resulted in a marked but transient stimulation of insulin release that was not accompanied by a demonstrable increase of glucose oxidation. Galactose and 3-O-methylglucose had no effect on glucose oxidation or on insulin release. The results are consistent with a metabolic model of the beta-cell recognition of glucose as insulin secretagogue and with the assumption that Ca(2+) or Na(+) deficiency, or the addition of adrenaline or diazoxide, inhibit insulin release at some step distal to stimulus recognition. In addition the results suggest that these conditions create a partial metabolic block of glycolysis in the beta-cells. Hence the interrelationship between the processes of stimulus recognition and insulin discharge may involve a positive feedback of secretion on glucose metabolism.

Iodoacetamide-induced sensitization of the pancreatic beta-cells to glucose stimulation

At a glucose concentration of 3mm or less, iodoacetamide had no effect on the release of insulin from microdissected pancreatic islets of ob/ob-mice. At higher glucose concentrations, iodoacetamide exerted both an initial stimulatory and a subsequent inhibitory action. When islets were perifused with 1mm-iodoacetamide and 17mm-glucose the inhibitory action predominated after about 15min of transient stimulation. With decreasing concentrations of iodoacetamide the stimulatory phase was gradually prolonged, and with 0.003-0.1mm-iodoacetamide stimulation only was observed for 75min. Prolonged stimulation was also noted after a short pulse of iodoacetamide. Similar responses to 0.1mm-iodoacetamide were observed with islets from normal mice. With islets from ob/ob-mice the effect of 0.1mm-iodoacetamide was reproduced with 0.1mm-iodoacetate, whereas 0.1mm-acetamide had no apparent effect. Iodoacetamide increased the V(max.) of glucose-stimulated insulin release without altering the apparent K(m) for glucose. Leucine, glibenclamide or theophylline could not replace glucose in this synergistic action with iodoacetamide. Iodoacetamide rather inhibited the insulin-releasing action of theophylline. Iodoacetamide-induced potentiation of the glucose-stimulated insulin release was rapidly and reversibly inhibited by mannoheptulose, adrenaline, or calcium deficiency. The potentiating effect on insulin release was not paralleled by effects on glucose oxidation or on islet fructose 1,6-diphosphate. However, the inhibitory action of iodoacetamide might be explained by inhibition of glycolysis as evidenced by an inhibition of glucose oxidation and a rise of fructose 1,6-diphosphate. The results support our previous hypothesis that thiol reagents can stimulate insulin release by acting on relatively superficial thiol groups in the beta-cell plasma membrane. Glycolysis seems to be necessary in order for iodoacetamide to stimulate in this way.